Les Cheneaux Watershed Council
…protecting, conserving and restoring our water-based resources.
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What is Eurasian watermilfoil?
Eurasian watermilfoil, Myriophyllum
spicatum ,is a broadleaf plant typically
having 12-21 pairs of leaflets. It is an
invasive species introduced into the lakes in
1950's. A similar but native and noninvasive
species, Myriophyllum sibiricum, or
Northern watermilfoil, is also found in the
Great Lakes region.
Because it is widely distributed, difficult to
control, and drastically alters local ecology,
Eurasian watermilfoil (hereafter referred to
as milfoil) is considered to be the most
problematic water dwelling plant in our
area. Milfoil forms dense mats of vegetation
on the surface of the water. These mats
interfere with recreational boating, fishing, swimming and water
skiing.
These stagnant mats can also create good habitat for mosquitoes
and can alter the water's chemistry and quality. Dense milfoil mats
alter water quality by raising pH, decreasing oxygen under the mats,
and increasing temperature. These factors can increase the
availability of nutrients for algal growth in the lake, can in turn would
reduce the lake’s water clarity.
Milfoil mats can rob oxygen from the water by preventing the wind
from mixing the oxygenated surface waters to deeper water. Dense
mats of vegetation can also increase the sedimentation rate by
trapping sediments. Monoculture milfoil stands provide poor habitat
for waterfowl, fish, and other wildlife. Significant rates of plant
sloughing and leaf turnover, as well as the decomposition of high
biomass at the end of the growing season, increase the internal
loading of phosphorus and nitrogen to the water column.
Eurasian watermilfoil,
Myriophyllum spicatum
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Growth Habitat
Eurasian watermilfoil exhibits an annual growth pattern. In the spring,
shoots begin to grow rapidly as water temperatures approach
60F/15C degrees. Milfoil also starts spring growth sooner than native
aquatic plants and can shade out these beneficial plants. When they
near the surface, shoots branch profusely, forming a dense canopy.
Typically, plants flower upon reaching the surface. After flowering,
plant biomass declines as the result of the fragmentation of stems.
During fall, plants die back to the root crowns, which sprout again in
the spring. Milfoil is winter-hardy, able to overwinter in frozen lakes
and ponds in northern states and Canada. When milfoil invades new
territory, typically the species diversity of aquatic plants
declines. While some species of waterfowl will eat milfoil, it is not
considered a favored food source.
Milfoil is able to reproduce successfully and rapidly through the
formation of plant fragments. In the late summer and fall the plants
become brittle and naturally break apart. These fragments can float
to other areas, sink, and start new plants; a process known as
autofragmentation. Milfoil will also grow from fragments created by
boaters, wind and water action, or other disturbances during any time
of year. A new plant can start from a tiny piece of a milfoil plant. This
is why milfoil can so easily be transported from lake to lake on boat
trailers or fishing gear. Once established in its new home, water
currents may carry milfoil fragments and start new colonies within the
same waterbody. These fragments grow roots, stems, and leaves as
they float along in water. Eventually they attach and take root in the
bottom sediments of ponds, lakes, or slow waters of streams and
ditches. Rooted plants also spread by sending out stolons to produce
new plants. The stored seeds in lake sediments also start new plants.
Once introduced, milfoil may spread rapidly. In Currituck Sound,
North Carolina, milfoil was first reported in 1965 when approximately
100 acres were densely infested and 500 to 1000 acres were lightly
infested. A year later 8000 acres were heavily infested and 66,000
acres had some milfoil plants. Nine years later, over 80,000 acres
were infested with Eurasian watermilfoil Milfoil control is a long term
effort. For example, a milfoil control program has been ongoing at
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Lake George, NY since 1985. To date over seven million dollars have
been spent to manage the Lake George milfoil infestation. Dozens of
other lakes have similar long-term infestation and expense histories.
Management
Once milfoil becomes established within a waterbody, it is difficult or
impossible to remove, however, due to decades of university, state
and federal research and experience with milfoil in the US and
Canada, several management methods have been developed.
Control methods include harvesting, rotovation (underwater
rototilling), installation of bottom barriers, hand pulling, dredging,
biological controls and use of aquatic herbicides.
Mechanical Control Methods
Because this plant spreads readily through fragmentation,
mechanical controls such as cutting, harvesting, and rotovation
(underwater rototilling) should be carefully considered because
resulting plant fragments may easily regrow or be carried
downstream to create new infestations. It is wise to apply systemic
herbicides prior to harvest so plant fragments do not grow new plants.
Cutting
Cutting is similar to harvesting except cut plants are not picked up
from the water by the cutting machine. This method is not
recommended.
Harvesting
Harvesting can be compared to underwater lawn mowing. Plants
are cut generally 5-to-8 feet below the water's surface, collected by
conveyer, and stored until disposal on land. Harvesting removes
surfacing mats and creates open areas of water. However, because
of its rapid growth, milfoil generally needs to be harvested twice
during the growing season.
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Rotovation
The British Columbia Ministry of Environment developed a barge
mounted rototilling machine called a rotovator to remove milfoil roots.
Underwater tiller blades churn up to 8 inches into the sediment and
dislodge buoyant milfoil roots. Floating roots may then be collected
from the water. Control with rotovation, generally extends 2 or more
growing seasons.
Herbicides
US EPA-approved aquatic herbicides can selectively target milfoil.
When properly applied these products are cost effective and can
reduce populations of this weed to the point where other options such
as aquatic weevils may be useful for long term control.
The two most typically applied materials are 2,4 D, trade name
Aquacide) and fluridone (trade name Sonar). These products act
differently, and are most effective in different environments.
Consequently, one must consider several factors such as time of
year, water movement, area and depth to be treated, and government
regulations before selecting the product of choice. Moreover, different
regulations apply to Great Lakes waters vs inland lakes.
▪ Fluridone (Sonar) is a systemic herbicide that affects the ability
of plants to photosynthesize. Visual symptom of fluridone action is
bleaching or the development of chlorosis on the growing points of
the plant. It is used at extremely low rates (10 to 20 parts per billion),
but requires a long contact time of four to six weeks and is most
effective when applied to beds greater than 5 acres in size.
Symptoms are seen on the plant, after 7-10 days and results typically
last a year. Due to the long contact period needed, this material is
best applied to beds with little water movement or to whole lake
applications.
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▪ 2,4-D (Tradenames of Aquacide, Navigate and Dow DMA 4) is
specific to broad leaf plants like the milfoil and will control the weeds,
including roots, when applied during active growth. Spring to early
summer applications provide quicker results, with less dead
vegetation and a longer benefit in the current year. The full effect of
mid summer to early fall applications may not be evident until the
following spring. Herbicide absorption is quite rapid, occurring in a
few hours under good growing conditions. Once absorbed, the
herbicide moves to and accumulates at growing parts of root and leaf.
Growth at these points is halted and the weed dies. Visible effects
occur within 7 to 10 days as a curling of stems and browning of leaf
tips. Full effect of application is evident in 3 to 5 weeks.
▪ The above products have application procedures, water use
precautions and permits which need to be applied and understood
before use.
Light Barriers
Localized control in swimming areas and around docks can be
achieved by covering the sediment with a opaque fabric which blocks
light from the plants.
Biological Controls
A plant pathogenic fungus Mycoleptodiscus terrestris has been
shown to significantly reduce milfoil biomass in laboratory studies. A
commercial biotechnology firm spent several years developing this
fungus as a biological tool to control milfoil, but was unable to achieve
control of the plant in field settings. The US Army Corps of Engineers
and entrepreneurs are continuing research on plant pathogens for
milfoil control
A naturally occurring aquatic weevil, Euhrychiopsis lecontei, feeds
preferentially on milfoil and has been found effective in controlling
light to moderate infestations. The weevil completes all life stages
fully submersed, and the larvae are stem miners. Eggs are laid on
milfoil meristems, and the larvae eat and bore down through the
stem, suppressing plant growth. Although this method has proven
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successful in our Cedarville trials, the weevils have been
overwhelmed by the dense milfoil population that exists in our waters
today.
What can you do?
Contact our Les Cheneaux Watershed Council with questions:
www.lescheneauxwatershed.org.
Watch for and read future Watershed Council publications on milfoil
management. Check the Council website often this next year to learn
how and where the milfoil control efforts are being conducted.
Cooperate with any directions which are given regarding restrictions
which may be needed to ensure effectiveness of management sites.
Do not transport milfoil from lake to lake. Always wash weeds off your
boat trailer before and after launching.
Clean up milfoil fragments from your boat or beach to help prevent
further growth
Be patient, understand that milfoil has been in existence in the lakes
of the United States for several decades. There is no clear way to
fully remove this species, however, there are several known methods
to help manage growth. The Les Cheneaux community is committed
to implement the methods which are most effective in our area.
References:
Department of Ecology, State of Washington, Non-native Invasive
Freshwater Plants
Center for Aquatic and Invasive Plants, University Of Florida IFAS,
Eurasian watermilfoil
Aquatechnex company literature
Cooperative Extension, University of Nevada,
Eurasian watermilfoil.